As you can see, the variable sys has been defined twice. Inside the <procs> block a variable ${sys} has been used, which then were interpolated into the value of sys defined inside the <table> block, not the sys variable one level above. If sys were not defined inside the <table> block then the "global" variable sys would have been used instead with the value of "unix".

Variables inside double quotes will be interpolated, but variables inside single quotes will not interpolated. This is the same behavior as you know of Perl itself.

In addition you can surround variable names with curly braces to avoid misinterpretation by the parser.